Recent work in the area of crossed-field devices has been directed to improving the output of high power microwave (HPM) sources. Crossed-field devices of particular interest include relativistic magnetrons and ubitrons. Further, improvement of the crossed-field devices has been through the use of conventional transparent cathodes. Generally speaking, a conventional transparent cathode consists of individual emitters periodically arranged about a fixed radius.
Magnetrons are widely used as powerful and compact sources for the generation of high power microwaves in a variety of applications. Such applications can include, but are not limited to, microwave ovens, telecommunications equipment, lighting applications, radar applications, and military and weapons applications, for example.
A typical conventional magnetron structure is a coaxial vacuum diode with a cathode having a solid cylindrical surface and an anode consisting of an even number of cavities forming an azimuthally periodical resonant system. In many designs, resonator cavities of various shapes are cut into the internal surface of the anode, for example, in a gear tooth pattern. During operation, a steady axial magnetic field fills the annular vacuum region between the cathode and anode, and a voltage is applied between them to provide conditions for microwave generation. TE-type eigenmodes of the resonant system are used as operating waves Two types of oscillations may be used, the π-mode (with opposite directions of electric field in neighbor cavities) and the 2π-mode (with identical directions of electric field in all cavities). The frequency of the generated microwaves is based in part on the number and shape of the resonator cavities, and the design features of the anode and cathode.
A cross-sectional view of a conventional A6 magnetron modeled using “MAGIC” particle-in-cell (PIC) code is illustrated in FIG. 1. As shown, a conventional magnetron comprises an anode 10, a cathode 20, which is a solid cylindrical structure, and resonator cavities 15. In this example, a waveguide 40 is located in one of resonator cavities 15 in order to extract the generated microwaves. A dielectric can also be present to extract the generated microwaves. There are other ways known to those skilled in the art for extracting the microwaves as well, such as, for example, axially using diffraction output.
Electrons emitted from the cathode 20 form a solid flow drifting around cathode 20 with velocity determined by the applied voltage and magnetic field. When the azimuthal phase velocity of one of eigenmodes of the resonant system is close to the azimuthal drift velocity of the electrons, energy of electrons is transferred to this electromagnetic wave. As the wave gains energy, fields of the wave back-react on the electron charge cloud to produce spatial bunching of the electrons, which in turn reinforces the growth of the wave.
Magnetrons are either of the hot (thermionic) cathode type, which typically operate at voltages ranging from a few hundred volts to a few tens of kilovolts, or of the cold cathode type, with secondary electron emission or explosive emission, the latter of which are typically used in relativistic magnetrons, which operate at high voltage (hundreds kilovolts) and enable the generation of very high power microwaves.
As indicated, it is known that use of the transparent cathode can significantly enhance radiation characteristics of relativistic magnetrons. A conventional transparent cathode is depicted by way of example in each of FIG. 2 and FIG. 3, with each example including a discrete number of thin explosive electron emission regions arranged azimuthally at a fixed radius corresponding to the radius of a traditional solid cathode. This arrangement allows the azimuthal wave electric field to go to zero on-axis, as opposed to going to zero on the surface of the solid cathode. This provides both cathode and magnetic priming, in addition to providing a much stronger wave electric field in the sheath region.
As shown schematically in FIG. 2, the cathode 200 has a thin-walled cylindrical body 210 which includes a number of separate strip-shaped emitter regions 220 supported at one end of the thin-walled body 210 and open at the end of the emitter regions. The emitter regions 220 are consecutively disposed around a longitudinal axis of the cathode body 210 such that an imaginary envelope surface surrounding the emitter regions 220 forms a substantially hollow cylindrical structure. The emitter regions 220 are typically spaced relative to each other at intervals around the perimeter of the cathode body 210. Thus, empty regions (openings) 225 between consecutive (e.g., adjacent) emitter regions 220 are formed. The empty regions 225 permit the passage of electromagnetic field therethrough such that the field “penetrates” the cathode 200 up to the longitudinal axis of the cathode body 210. Accordingly, the cathode 200 is referred to as a “transparent” cathode.
As also shown in FIG. 2, the emitter regions 220 are longitudinally oriented and substantially parallel to one another. The number, azimuthal position with respect to anode resonant cavities and configuration of the emitter regions 220 can be selected so as to achieve certain operating characteristics of the magnetron.
FIG. 3 differs from FIG. 2 in depicting cylindrical rather than strip-shaped emitter regions. As shown schematically in FIG. 3, the cathode 300 has a thin-walled cylindrical body 310 which includes a number of separate emitter regions 320 supported at one end of the thin-walled body 310 and open at the distal end of the emitter regions. The emitter regions 320 are consecutively disposed around a longitudinal axis of the cathode body 310 as described in connection with FIG. 2. The emitter regions 320 are typically spaced relative to each other at intervals around the perimeter of the cathode body 310. Thus, empty regions (openings) 325 between consecutive (e.g., adjacent) emitter regions 320 are formed. The empty regions 325 permit the passage of electromagnetic field therethrough such that the field “penetrates” the cathode 300 up to the longitudinal axis of the cathode body 310.
As also shown in FIG. 3, the cylindrically shaped emitter regions 320 are longitudinally oriented and substantially parallel to one another. The number, azimuthal position with respect to anode resonant cavities and configuration of the emitter regions 320 can be selected so as to achieve certain operating characteristics of the magnetron.
Another crossed-field device, an ubitron (not shown), can also be used in connection with the transparent cathode of FIG. 2 or FIG. 3 for providing improved high power microwave output. The ubitron has been studied in both simulations and experiments. The ubitron is a simple device, including a transparent cathode in a pipe with an axial magnetic field. In the ubitron, the electron sheath flows through a periodic transverse magnetic field, similar to Bekefi's smooth-bore magnetron. Unlike Bekefi's device, however, the ubitron requires no external permanent magnets.
In either of the relativistic magnetron or ubitron, the emitter regions (cathode strips) of the conventional transparent cathode are supported at only a single end by the thin-walled cylindrical body, and can therefore deform over time. Deformation of the cathode strips can lead to a decrease in performance of the magnetron or ubitron. Further, when cathode strips are very long or thin, they can be unable to support their own weight over their length. In repetitive pulse or continuous wave type magnetrons, electron bombardment can heat up the cathode strips and further decrease their mechanical strength, particularly at their longer length or if the cathode strips are relatively thin. In magnetrons that operate at high currents, longitudinal currents flowing in the cathode strips and the magnetic forces between the cathode strips can also contribute to their deformation.
Although the known transparent cathode can provide advantages over the conventional solid cathode, they may not address many of the deficiencies and/or desirable features noted above. By way of example, and not limitation, the conventional approaches may not provide sufficient mechanical strength to the transparent cathode in the case of very long or thin cathode strips since the cathode strips are only supported at one end. Moreover, cathode strips of the known transparent cathode can experience warping since the strips are only supported on one end. The individual cathode strips can carry kilo-amperes of current that can induce severe ohmic heating of the metal, thereby degrading its mechanical integrity. Further, the currents in the individual cathodes also generate magnetic fields, and the forces due to these magnetic fields may warp the cylindrical profile of the cathode strips. These detrimental occurrences can be avoided with the disclosed novel transparent eggbeater cathode.
Thus, there is a need to overcome these and other problems of the prior art and to provide a transparent cathode for a magnetron, ubitron, or the like having fully supported cathode strips.